Sharp edged metal structures, such as cutting tools, are typically comprised of a body having a sharp edge defined by at least one surface of the body. For example, a punch typically has a sharp edge or point defined by a conical surface. Other sharp edges may be linear or curved and be defined by the intersection of two or more surfaces. In some cases, such as the case of forming tools, the surfaces may be of irregular contour. In other cases, such as in the case of drills, the surfaces may be of a uniform contour.
Many techniques have been employed for lengthening the life of cutting tools. Such techniques include the manufacture of the tool out of a long life material such as high speed steels or cemented tungsten carbide. Surface hardening techniques used to improve tool life such as case hardening have been commonly applied, but such techniques do not provide sufficient improvement in tool life to meet the current needs of industry. Hard coatings such as chemically vapor deposited titanium carbide and titanium nitride have also been tried with no success because of the degradation of the mechanical properties and deformation of the parts by high temperatures (.about.900.degree. C.) used during the deposition process.
The thermochemically deposited coatings described in U.S. Pat. No. 4,008,976 issued Feb. 22, 1977 to Holzl have also been tried to prolong the tool life. In that patent, a coating of very substantial thickness is deposited on a tool body and a cutting edge is subsequently machined in the deposited layer. Unfortunately, machining of such high hardness material may be difficult and may, in many instances, undesirably increase the cost of the tool. Although very thin coatings of this nature have also been attempted on cutting tools and the like, such coatings have not been successful. This is because, in many instances, deposition of such coatings requires very high temperature, causing degradation of the mechanical properties and deformation of the parts. Differences in coefficients of thermal expansion between the substrate and the coating also results in poor coating adhesion.
U.S. Pat. No. 4,162,345, issued July 24, 1979 to Robert A. Holzl, discloses a method for producing deposits characterized by a structure which is free of columnar grains and instead consists essentially of fine, equiaxial grains. These deposits have unusually high hardness and tensile strength. However, the Holzl '345 patent discloses use of temperatures varying from 650.degree. C., to l,lOO.degree. C., which are high enough to degrade the mechanical properties and deformation (or mechanical distortion) of metallic substrates. The material of Holzl '345 patent is a hard metal alloy, consisting primarily of tungsten and carbon. X-ray diffraction analysis of the '345 alloy shows that the deposit is akin to tungsten but with a very finely dispersed carbide, probably in the form of WC.
U.S. Pat. No. 4,427,445, issued Jan. 24, 1984 to Robert A. D Holzl, et al. also discloses a hard fine grained material which can be produced by thermochemical deposition, but at temperatures lower than those described in the example of the '345 patent. Thus, where there are large differences in the thermal coefficients of expansion between the substrate material and the coating material, the '445 methodology reduces adhesion problems and problems associated with mechanical distortion, metallurgical transformation or stress relief of the substrate. The material of the 445 'Holzl, et al. patent is a tungsten carbon alloy consisting primarily of a two phase mixture of substantially pure tungsten and an A15 structure.
U.S. Pat. No. 3,368,914, discloses a process for adherently depositing tungsten carbide of substantial thickness on steel and other metal substrates. The process involves first diffusing another metal on the surface of the substrate to relax the thermal expansion coefficient zone of the metal substrate. The carbide coating is then deposited on the diffused surface by CVD. The process claims it is preferable to diffuse the metal forming the carbide into the substrate. In one embodiment of the claimed process, a thin layer of tungsten is deposited on the metal surface using 600.degree.-1000.degree. C. temperature. After coating tungsten, the temperature is increased to approximately 1000.degree.-1200.degree. C. and held there for a significant period of time to permit diffusion of tungsten into the metal. The diffused surface is then coated with tungsten carbide using WF.sub.6, CO and H.sub.2. In the alternative embodiment, a pack diffusion technique is used for achieving diffusion of tungsten into metal. Temperature ranging from 1000.degree.-1200.degree. C. is used in the pack diffusion step. The diffused metal surface is then coated with tungsten carbide. Since a temperature ranging from 1000.degree.-1200.degree. C. is used during the process, the '914 process is not suitable for providing erosion and wear resistance coatings on various metallic substrates without severely distorting and degrading their mechanical properties.
U.S. Pat. No. 3,389,977, discloses a method of depositing substantially pure tungsten carbide in the form of W.sub.2 C, free from any metal phase. Pure W.sub.2 C is deposited on a substrate by reacting WF.sub.6 and CO. The substrate is heated to a temperature in excess of 400.degree. C. The adherence of W.sub.2 C to steel is improved by first cleaning the surface and then depositing with a thin film of tungsten followed by W.sub.2 C using a temperature ranging from 600.degree.-1000.degree. C. Since initial deposition of tungsten is conducted at or above 600.degree. C., the '977 process is not suitable for providing erosion and wear resistance coating on various carbon steels, stainless steels, nickel and titanium alloys without severely degrading their mechanical properties. Additionally pure W.sub.2 C deposited according to the teachings of the '977 patent consists of columnar grains. The '977 patent does not describe a process for depositing W.sub.2 C coating in non-columnar fashion.
U.S. Pat. No. 3,574,672 discloses a process for depositing W.sub.2 C by heating a substrate to a temperature between 400.degree.-1300.degree. C. The process described in this patent is essentially the same as disclosed in U.S. Pat. No. 3,389,977.
U.S. Pat. No. 3,721,577 discloses a process for depositing refractory metal or metal carbides on ferrous and non-ferrous base materials heated to at least 1050.degree. C. The metal carbides are deposited using halide vapors of the metal along with methane and hydrogen.
U.S. Pat. No. 3,814,625 discloses a process for the formation of tungsten and molybdenum carbide by reacting a mixture of WF.sub.6 or MoF.sub.6, benzene, toluene or xylene and hydrogen. The process is carried out under atmospheric pressure and temperatures ranging from 400.degree.-1000.degree. C. An atomic ratio of W/C in the gaseous mixture varying from 1 to 2 is required to yield W.sub.2 C. The process also suggests that for some substrates such as mild steel, it is advantageous in providing better adhesion to deposit a layer of nickel or cobalt prior to tungsten carbide deposition. The process also claims the formation of a mixture of tungsten and tungsten carbide in the presence of large proportions of free hydrogen. The mixture of W and W.sub.2 C coating deposited according to the teaching of the '625 patent consists of columnar grains. The '625 patent does not disclose a process for depositing a mixture of W and W.sub.2 C in non-columnar fashion.
British Pat. No. 1,326,769 discloses a method for the formation of tungsten carbide by reacting a mixture of WF.sub.6, benzene, toluene or xylene and hydrogen under atmospheric pressure and temperatures ranging from 400.degree.-1000.degree. C. The process disclosed in this patent is essentially the same as disclosed in U.S. Pat. No. 3,814,615.
British Patent No. 1,540,718 discloses a process for the formation of W.sub.3 C using a mixture of WF.sub.6, benzene, toluene or xylene and hydrogen under sub-atmospheric pressure and temperature ranging from 350.degree.-500.degree. C. An atomic ratio of W/C in the gaseous mixture varying from 3-6 is required to yield W.sub.3 C. The coating deposited according to the teaching of British Pat. No. 1,540,718 consists of columnar grains. The British '718 patent does not teach a process for depositing a non-columnar coating.
Although the methods of the Holzl patents cited above have been useful in producing fine-grained tungsten/carbon alloys containing mixtures of W and WC, and W and A15 structure, and the methods described in other cited patents have been successful in producing columnar W.sub.3 C or W.sub.2 C or mixtures of W and W.sub.2 C, no one has yet disclosed a method for producing extremely hard, fine-grained, non-columnar tungsten-carbon alloys with substantially layered microstructure containing mixtures of tungsten and tungsten carbide in the form of W.sub.2 C or W.sub.3 C or a mixture of W.sub.2 C and W.sub.3 C. Such alloys would be especially useful since the presence of W.sub.2 C and/or W.sub.3 C in non-columnar and substantially layered microstructure would contribute to both the hardness and the tensile strength of the deposited materials.
In co-pending U.S. application Ser. No. 07/092,809, a method and coating are described comprising a non-columnar, fine grained, and having a substantially layered microstructure deposit of tungsten carbide in the form W.sub.2 C, W.sub.3 C or mixtures of W.sub.2 C and W.sub.3 C. In co-pending U.S. application Ser. No. 07/153,738 such coatings are described in which an intermediate layer of substantially pure tungsten is used between the substrate and the tungsten carbide outer layer to confer additional erosive, abrasion and wear resistance characteristics on the composite coating system. The present invention is an improvement on such prior described coating systems wherein such systems are applied at low temperatures to a sharp edged metal structure such as a cutting tool or the like without distorting, deforming and degrading their mechanical properties. It has been found that by controlling the thickness of such coating systems, they can be applied without significantly altering the original machined configuration of the sharp edged metal structure. It has also been found that such coating systems can confer substantial abrasion, erosion and wear characteristics on such structures.